Rechargeable batteries can be used in many applications. One such application is the use of batteries in hybrid or fully electric vehicles. Within these vehicles a plurality of individual “cells” are arranged in series in order to build a battery stack having a desired output voltage. A large number of cells may be arranged in series such that, for example, the total potential difference developed across the battery stack is in the order of several hundred volts. Each cell typically only has a potential difference of a few, say 2 to 4, volts developed across it. Although the cells are similar, they are not identical, so repeated charging and discharging cycles may mean that the voltage developed across some cells within the stack are different to those developed across other cells within the stack. Ideally the voltage across each individual cell, or at least small groups of cells, would be monitored such that the cells could be temporarily removed from a charging process if their terminal voltage gets too high or alternatively if the cell temperature becomes unduly elevated. It is also possible to preferentially discharge cells to reduce their voltage. Whilst it is feasible to build a single battery monitoring apparatus that can operate across the entire voltage range, for example 0 to 400 volts, developed across a stack, this tends to be an expensive option.